CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is a well-known media access control (MAC) protocol used in networks having a shared carrier medium (e.g. wireless local area networks (WLANs)). Under this protocol, nodes in the network sense the transmissions made by other nodes over the shared carrier medium and defer their own transmissions until the network becomes idle. In some examples, each node may wait for a random period of time, either before (in the case of IEEE 802.15.4) or after (in the case of IEEE 802.11) a clear channel assessment (CCA), so as to reduce the probability of two or more nodes, sensing that the network has become idle, beginning to transmit at the same time, causing a collision.
In a wireless network, it may often be the case that nodes are unable to sense transmissions made by other nodes. This may be because the nodes are not within range of each other's transmissions, or because of interference or shielding of transmissions caused by the environment (e.g. building) in which the nodes are located.
The hidden node problem (HNP) describes a situation in which transmitter nodes in a network are each visible to a receiver node, but in which the transmitter nodes are not visible to each other. Under the HNP a first transmitter node, sensing no activity on the network, may begin a transmission while the receiver node is already receiving a transmission from another transmitter node that is hidden from the first transmitter node. This causes a collision at the receiver node. The carrier sense method under CSMA can therefore fail to avoid collisions under this scenario.
The exposed node problem (ENP) describes a situation in which the transmissions of two transmitter nodes are visible to each other, but in which the receivers of those transmissions are not in range of each other. This can give rise to false collisions at the transmitter nodes, since the CSMA protocol will prevent them from making simultaneous transmissions to their respective receiver nodes, even though this would not in fact lead to a collision at the receiver nodes. The ENP can often arise in networks using the dense tree topology, in which the spatial distribution of the nodes is not correlated with the communication patterns in the network. For example, it may be that the transmitter nodes, although physically located close to each other, may be unrelated routers that serve different branches of the network tree (they have different parent nodes). In some cases, the transmitter nodes, although operating under the same communication standard (e.g. IEEE 802.15.4), may not even belong to the same network.
An example of a Collision Avoidance (CA) protocol that is used with CSMA/CA employs a handshake procedure prior to data transmission in which RTS (Request to Send) and CTS (Clear to Send) messages are exchanged by a transmitter node and the intended receiver node in order to establish that the receiver node is available and ready.
The use of RTS and CTS messages can alleviate the HNP, since the receiver node only sends a CTS message to one transmitter node at a time. Therefore, a transmitter node must wait until it is sent a CTS message from the receiver node before it begins its own transmission, even though it may sense no activity over the carrier medium. Transmitter nodes that are hidden from each other rely on the CTS messages to prevent collisions involving their transmissions at the receiver node.
The use of RTS and CTS messages can also alleviate the ENP. When a first transmitter node hears an RTS message from a neighbouring second transmitter node, but does not then hear a CTS message from the receiver node to which the RTS message was sent, the first transmitter node can deduce that it is an exposed node. Based on this, the first transmitter can then make transmissions to another receiver node in the knowledge that they will not cause collisions at the receiver node to which the initial RTS message was sent.
Although RTS and CTS messages can reduce the occurrence of collisions in a wireless network, they lead to additional congestion in the network and are generally enabled only for large frame sizes. The use of RTS and CTS messages is therefore made optional in the IEEE 802.11 standard.
In some wireless networks, especially those having relatively small maximum transfer unit (MTU) sizes, the additional overhead imposed by the use of RTS/CTS messages may not warrant their use. For instance, the IEEE 802.15.4 standard defines a MTU of 127 bytes. This is especially the case in crowded networks including a large number of densely spaced nodes.
The IEEE 802.15.4 standard specifies the physical layer and MAC layer for low rate wireless personal networks (WPANs). The ZigBee standard is based on IEEE 802.15.4-2003 version and was developed for use in low rate, low power, secure applications. These standards are suitable for use in wireless sensor networks (WSNs) in the home and building automation domain (Smart Buildings). The use of RTS and CTS messages may be too onerous for these kinds of networks, in which low power usage is a requirement. Further, the small MTU sizes in WSN applications may not justify using RTS/CTS messages. This is especially true in the context of the internet of things (IoT), in which the network is anticipated to be extremely crowded with a huge number of nodes.